vectorize a function with multiple variables

Question

Consider the arbitrary function:

function myFunc_ = myFunc(firstInput, secondInput)
    myFunc_ = firstInput * secondInput;
end

Now imagine I want to map the above function to an array for the first input firstInput, while the second input secondInput is constant. For example, something like:

firstVariable = linspace(0., 1.);

plot(firstVariable, map(myFunc, [firstVariable , 0.1]))

where 0.1 is an arbitrary scalar value for the secondInput and firstVariable array is an arbitrary array for the firstInput.

I have looked into the arrayfun() function. However, I don't know how to include the constant variable. Plus it seems like the syntax between MATLAB and Octave are different, or maybe I'm mistaken. It is important for me to have a cross-compatible code that I can share with colleagues.

Maybe I misunderstand what you mean by map(fun, argv), but wouldn't plot(firstVariable, myFunc(firstVariable, 0.1)) suffice? — Vicky
– Vicky, Commented Mar 14, 2021 at 22:10
@Vicky the myFunc(firstVariable, 0.1)) returns a scalar in Octave. I don't know what it does on MATLAB though! — Foad S. Farimani
– Foad S. Farimani, Commented Mar 14, 2021 at 22:22
myFunc(firstVariable, 0.1) returns a vector in MATLAB!! What happens if you substitute an elementwise .* for the * in your function definition, does that make any difference? (octave.org/doc/v6.2.0/Broadcasting.html) — Vicky
– Vicky, Commented Mar 14, 2021 at 22:40

Tasos Papastylianou · Accepted Answer · 2021-03-14 23:04:03Z

3

Assuming in the original function you were multiplying two scalars and you want to vectorise, then

function myFunc_ = myFunc(firstInput, secondInput)
    myFunc_ = firstInput .* secondInput;
end

should work just fine.

Then plot it directly:

plot( firstVariable, myFunc(firstVariable , 0.1) )

answered Mar 14, 2021 at 23:04

Tasos Papastylianou

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8 Comments

Vicky Over a year ago

That's what I'd suspected from reading the docs!

Foad S. Farimani Over a year ago

No, I'm afraid the actual code is not as simple as the example given in the original question. I think I have found the right syntax that works on Octave. I will post it below for those who end up here.

Foad S. Farimani Over a year ago

Maybe I am making a mistake here, but I'm implementing equation 26 on this paper. And I have indeed used the elementwise arithmetic operators in the actual code.

Cris Luengo Over a year ago

@Foad: There's no reason you wouldn't be able to trivially vectorize that equation. You'd compute each of the three components first, say r1, r2 and r3, which would all be arrays, and then you compute max(max(r1,r2),r3) to find the element-wise max of the three. Computing max([r1,r2,r3]) could result in a scalar.

Cris Luengo Over a year ago

@Foad: Change your tmp0 function as I indicate in my comment above.

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Foad S. Farimani · Accepted Answer · 2021-03-15 21:43:49Z

1

I'm afraid the arbitrary examples given in the original question were too simplified and as a result, they do not represent the actual issue I'm facing with my code. But I did manage to find the right syntax that works inside Octave:

plot(firstVariable, arrayfun(@(tempVariable) myFunc(tempVariable, 0.1), firstVariable))

basically the

@(tempVariable) myFunc(tempVariable, 0.1)

creates what is so-called an anonymous function and the

arrayfun(<function>, <array>)

maps the function over the given array.

answered Mar 15, 2021 at 21:43

Foad S. Farimani

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